I  found reference to the Summer Solstice as being referred to as ‘An Fheill-Eoinm’, on a website called ‘Celtic World’. However, I do know that one of the authors of the site is a SF writer; but I don’t know what her scholarship is. The Gaelic means, basically, John’s Feast and makes reference to it being the Christian St. John’s Feast Day.

Source:
http://www.applewarrior.com/celticworld/celticyear/quarters.html